The present invention relates generally to an endpoint detection method and apparatus, and more particularly to a method and apparatus that utilize a chelating agent to detect a polishing endpoint of a semiconductor wafer.
Semiconductor integrated circuits are typically fabricated by a layering process in which several layers of material are fabricated (i) on or in a surface of a wafer, or (ii) on a surface of a previous layer. This fabrication process very often requires layers to be fabricated upon a smooth, planar surface of a previous layer. However, the surface topography of layers may be highly uneven due to (i) areas which are higher than the remainder of the surface or (ii) an uneven topography of an underlying layer. As a result, a layer may need to be polished so as to present a smooth planar surface for the next processing step, such as formation of a conductor layer or pattern on the surface of another layer.
In general, a semiconductor wafer may be polished to remove high topography and surface defects such as crystal lattice damage, scratches, roughness, or embedded particles of dirt or dust. The polishing process typically is accomplished with a polishing system that includes top and bottom platens (e.g. a polishing table and a wafer carrier or holder), between which the semiconductor wafer is positioned. The platens are moved relative to each other thereby causing material to be removed from the surface of the wafer. This polishing process is often referred to as mechanical planarization (MP) and is utilized to improve the quality and reliability of semiconductor devices. The polishing process may also involve the introduction of a chemical slurry to facilitate (i) higher removal rates, and (ii) selective removal of materials fabricated upon the semiconductor wafer. This polishing process is often referred to as chemical mechanical planarization or chemical mechanical polishing (CMP).
In these polishing processes, it is often important to determine an endpoint of the polishing process. Over polishing (removing too much) of a conductive layer results in increased circuit resistance and potential scrapping of the semiconductor wafer. Since many processing steps have occurred prior to the polishing process, scrapping a semiconductor wafer during fabrication may result in a significant financial loss. Underpolishing (removing too little) of a conductive layer on the other hand leads to failure in isolating circuits and results in electrical shorts, which leads to rework (redoing the CMP process) which raises the cost of production. Thus, a precise endpoint detection technique is needed.
A typical method employed for determining endpoint in polishing systems is to measure the amount of time needed to planarize a first wafer, and then to run the remaining wafers for similar times. In practice this method is extremely time consuming, since operators must inspect each wafer after polishing. The wafers must be inspected because the removal rate may vary during the polishing of an individual wafer, thus making it extremely difficult to precisely control the polishing process.
Another method employed for determining the endpoint in polishing systems is to (i) form a polishing endpoint layer in the semiconductor device, and (ii) polish the semiconductor device down to the polishing endpoint layer. To this end, polishing systems detect when the polishing process reaches the polishing endpoint layer and terminate the polishing process in response to reaching the polishing endpoint layer. Various techniques have been used to detect when the polishing process reaches the polishing endpoint layer. For example, U.S. Pat. No. 5,668,063 to Fry et al. polishes a semiconductor device down to a tracer layer of detectable material. The polishing system of Fry determines that the tracer layer has been reached when a chemical element detector detects materials such as boron or phosphorous of the tracer layer have been removed by the polishing process.
In order to base endpoint detection upon directly detecting material of the tracer layer, the chemical element detector needs to accurately detect rather small amounts of the tracer layer material, or the polishing system needs to remove more of the tracer layer material in order to provide the chemical element detector with enough material for accurate detection. The above is also true if the material of the tracer layer is consumed as a reagent of a chemical reaction to be detected by the detector. In this case, the detector would need to be able to detect the effect of a small reaction, or the polishing system would need to remove more of the tracer layer in order to provide enough tracer material for a substantial reaction to occur.
Detectors capable of detecting small amounts of the tracer layer or detecting the effect of a small chemical reaction are more expensive than detectors capable of detecting larger amounts of the tracer layer or detecting the effect of a larger chemical reaction. Furthermore, the additional removal of the tracer layer in order to provide more tracer layer material for direct detection increases the risk of overpolishing especially when the topography of the tracer layer is highly uneven.
Thus, a continuing need exists for a method and an apparatus which accurately and efficiently detects when a polishing system polishes a semiconductor device down to a polishing endpoint layer.
In accordance with one embodiment of the present invention, there is provided a method of planarizing a semiconductor wafer having a polishing endpoint layer that includes a ligand. One step of the method includes polishing a first side of the wafer in order to remove the ligand from the wafer. Another step of the method includes determining that a chelating agent has bound the ligand due to the polishing step removing the ligand of the polishing endpoint layer. The method also includes the step of terminating the polishing step in response to determining that the chelating agent has bound the ligand.
Pursuant to another embodiment of the present invention, there is provided a method of planarizing a semiconductor wafer down to a predetermined distance from a semiconductor substrate of the wafer. One step of the method includes forming in the wafer a ligand that is at the predetermined distance from the substrate of the wafer. Another step of the method includes polishing a first side of the wafer in order to remove the ligand from the wafer. The method also includes the step of determining that a chelating agent has bound the ligand due to the polishing step removing the ligand from the wafer. Moreover, the method includes the step of terminating the polishing step in response to determining that the chelating agent has bound the ligand.
Pursuant to yet another embodiment of the present invention, there is provided an apparatus for polishing a semiconductor wafer down to a ligand of the wafer. The apparatus includes a polishing platen having a polishing surface, a wafer carrier, a slurry supply system, and a polishing endpoint detector. The wafer carrier is configured to (i) engage the wafer by the second side of the wafer and (ii) press the first side of the wafer against the polishing surface of the polishing platen. The slurry supply system is configured to apply a chemical slurry to the first side of the wafer which (i) facilities removal of the ligand from to the wafer, and (ii) receives the ligand removed from the wafer carrier. The polishing platen and the wafer carrier are configured to rub the first side of the wafer against the polishing surface in the presence of the chemical slurry in order to remove the ligand from the wafer. The polishing endpoint detector is operable to detect whether a chelating agent has bound the ligand due to the polishing platen removing the ligand from the wafer. Moreover, the polishing endpoint detector is operable to cause the polishing of the wafer to terminate in response to detecting that the chelating agent has bound the ligand.
It is an object of the present invention to provide an improved method and apparatus for determining an endpoint of a polishing process.
It is an object of the present invention to provide a new and useful method and apparatus for determining an endpoint of a polishing process.
It is also an object of the present invention to provide a method and apparatus which accurately and efficiently detect when a polishing system polishes a semiconductor device down to a polishing endpoint layer.
It is a further object of the present invention to provide a method and apparatus for determining that a polishing system has polished a device down to a polishing endpoint without removing the device from the polishing system.
The above and other objects, features, and advantages of the present invention will become apparent from the following description and the attached drawings.